The present invention relates to an antenna diversity system having a beam steering arrangement, and more particularly to an antenna diversity circuit, capable of generating a range of antenna beam patterns, and to a method of operating such a system. Applications of such a system include wireless data links in a business or domestic environment, and cordless and cellular telecommunications systems such as DECT (Digital Enhanced Cordless Telecommunications) and GSM (Global System for Mobile communication).
For convenience of description the present invention will be described with reference to an antenna diversity system used with a receiver, however it should be understood that the invention is equally applicable to use with a transmitter owing to the reciprocity between transmitting and receiving antennas.
In a complex propagation environment such as within a building, signal fading is a common occurrence. The fading is caused by the mutual interference of multiple signals arriving along propagation paths of different lengths. If the interference is destructive the received signal strength may be reduced below the level required for adequate reception.
Within a building there is often a significant difference in the direction of propagation of the signals, with the result that a receiver having an antenna which can be directed towards a good signal while rejecting other signals will experience less severe fading and therefore enjoy better quality reception.
The propagation environment within a building is typically varying rapidly, for example because of the movement of people in the vicinity of the propagation paths. Hence, for the potential improvement offered by a directional antenna to be realised it must be able to be steered rapidly, which therefore requires electronic steering.
Adaptive antenna systems comprising a plurality of antennas are known. For example, published PCT Application WO 95/33312 discloses an arrangement for selecting two antennas from a plurality of directional antennas using two receivers. A first signal quality metric is measured for a communication signal received from a first antenna, and a second signal quality metric is measured for the communication signal received from a second antenna. When the difference between the first and second signal quality metrics exceeds a threshold a receiver is coupled to a third antenna and a third signal quality metric is measured. Based on the three metrics a primary and secondary antenna are selected from which to receive the communication signal. Steering is realised in this system by changing the selected directional antenna. However, such known systems have the disadvantage of requiring at least two complete receivers which makes them relatively expensive.
An example of an antenna diversity arrangement requiring only a single receiver is provided in our PCT application WO 97/08774 (PHB 34000) which provides a transceiver having two antennas and an arrangement for selecting the antenna providing the strongest received signal. While providing some benefits, such an arrangement makes no attempt to use the antenna which is not selected, and cannot selectively reject multipath signals.
An object of the present invention is to effect antenna diversity in a manner which selectively rejects multipath signals.
According to a first aspect of the present invention there is provided an antenna diversity system comprising at least two antennas, each of the at least two antennas being coupled to beam forming means, and control means for controlling the beam forming means to provide at least two alternative predetermined antenna beam patterns.
According to a second aspect of the present invention there is provided a method of operating an antenna diversity system comprising at least two antennas, each of the at least two antennas being coupled to beam forming means, and control means for controlling the beam forming means, the method comprising operating the control means to provide at least two alternative predetermined antenna beam patterns.
The present invention is based upon the recognition, not present in the prior art, that a variety of beam patterns can be formed by an antenna diversity arrangement comprising a plurality of antennas. Each beam pattern is produced by one or more of the antennas being connected to a transceiver while the remainder of the antennas are terminated by an appropriate impedance. Only a single transceiver is required.